It is desirable to eliminate noise from FM radio signals to avoid distracting sounds in the audio broadcast. Automotive radios are particularly susceptible to noises because of their proximity to the engine ignition system which generously emanates radio signals. The spark of the ignition system often creates a very short duration impulse or spike known as a tic which is very disconcerting to the audio listener. It is already known to eliminate the tic by detecting its occurrence and blanking the noise by preventing radio signal passage to the speakers for the duration of the tic which is generally much shorter than 60 microseconds. Because this particular type of noise has such a short duration the interruption of the signal is not noticeable if that period is filled with an approximation of the correct signal. There are two critical aspects of such noise blanking: correctly and efficiently detecting the impulse noise and removing the noise in the optimal manner.
An example of a prior attempt to blank short duration pulses is shown in the U.S. Pat. No. 4,293,736 to Ogita which is conceptually like FIG. 1 herein. An input signal received at an antenna 10 is converted into an IF signal in a front end circuit 12 and is amplified in the IF amplifier 14. The signal is FM detected at detector 16 to generate an FM composite signal and a multiplex stereo demodulator 18 produces right and left channels which are fed through sample and hold circuits 20 to the receiver output terminals 22. To detect noise pulses a high pass filter 24 passes the ultrasonic portion of the FM composite signal to an automatic gain control circuit 26 and the resultant signal is averaged at an averaging circuit 28 and forwarded to a comparator 30 via a threshold 32. The non-averaged signal is fed to the comparator 30 via a delay 34. Thus a pulsating noise signal is compared to the average noise plus a fixed offset and produces a comparator output when the noise spike is high enough to exceed the threshold. That output triggers the sample and hold circuits 20 causing them to block any ensuing signal for the duration of the comparator output and instead holds the signal that was present when the spike was detected.
Problems arise with both the noise detection and the noise blanking aspects of Ogita. First, the high pass filter allows all high frequency signals including the signal of an adjacent channel to be considered as noise thereby giving rise to false noise signals. Second, the fixed offset or threshold makes the noise detector sensitive to any offset generated in prior stages in the circuit. Third, the AGC 26, which is required when the fixed offset is used, is a complex addition to the circuit and introduces a time constant to the circuit. Fourth, the sample and hold circuit 20 is placed after the demodulator 18. Since the demodulator contains a low pass filter to serve as a deemphasis circuit and to remove unwanted high frequencies from the audio signal, any noise pulse becomes stretched in the filter so that the signal must be blanked for a longer time to remove the noise. This obviously increases the difficulty of noise removal without audible disturbance.
Another common error in noise blanking is the attempted removal of the noise prior to the deemphasis circuit. This is illustrated in the U.S. Pat. No. 4,574,390 to Hirohashi et al. which shows a noise reduction circuit just after a separation circuit and before the deemphasis circuit. A sample and hold of the signal present when the noise occurs is effected there. It should be recognized, however, that under certain conditions there is little or no correlation between the audio signal and the instantaneous value of the wide bandwidth composite signal. At that point, the signal has much high amplitude, high frequency content superimposed on the audio signal. FIG. 2 shows an ideal audio signal as eventually output to the speakers. FIG. 3 shows the same signal before the deemphasis circuit filters of high frequency components. If the unfiltered signal is sampled at any point, there is a danger of sampling a high frequency component which is very different from the base band signal thus causing a noise rather than reducing noise. For example, the point desired to be sampled in the audio signal may be at point A of FIG. 2 but the actual point in the unfiltered signal may be point B of FIG. 3. To make matters worse, Hirohashi et al propose to alter the sampled signal according to the slope of the signal at the time of the noise signal. The high frequency content introduces very steep slopes not correlated with the base band signal slopes. Thus the sampled signal can be very wrong initially and then get much worse during the hold period. Complicated circuit are proposed in the patent to try to compensate for these additional problems.